New air-pollution limits for diesel engines have caused some manufacturers to adopt selective catalytic reduction (SCR) technology for reducing nitrogen oxides (NOx) in engine exhaust (e.g., diesel engine exhaust). The SCR process introduces (“injects”) an NOx reductant solution—typically a urea-water solution—into the hot exhaust gas. The reductant chemically reduces NOx into non-pollutant compounds at a catalyst substrate.
One type of prior art diesel engine exhaust system that employs an SCR process is illustrated in FIG. 1. As best shown in FIG. 1, a diesel engine 4 produces NOx as a component of an exhaust stream. The exhaust stream is directed from the engine 4 into the exhaust system 6, which includes a doser or injector section 8 in gaseous communication with an exhaust port of the engine 4 through an exhaust pipe 10. The doser section 8 includes a urea doser or injector 12 that is configured to inject a solution of urea (or other nitrogen oxides reductant) into the exhaust stream present in the doser section 8.
Referring still to FIG. 1, after the urea doser 12 injects urea into the exhaust stream, the exhaust travels through an exhaust pipe 14 to a catalytic section 16, where the urea and a catalyst within the catalytic section 16 react to reduce the nitrogen oxides into non-pollutant species. The reduced nitrogen oxides and remaining exhaust components are then directed out of the vehicle through an exhaust outlet pipe 18.
However, during typical operation of an engine, the conditions for the SCR process are not optimal (e.g., the exhaust is too hot or cold) and either too much, or too little, urea solution is introduced into the exhaust. When urea solution is introduced that does not undergo the SCR process (e.g., too much urea solution for the exhaust temperature), urea crystals will accumulate within the exhaust system, both on the interior surface of the exhaust pipe and at the port that introduces the solution into the exhaust. Build-up of urea crystals in the exhaust system detrimentally affects the performance of the exhaust system and also is indicative of an inefficiency in the SCR process—urea crystals represent both wasted urea solution and reduced SCR efficiency.
Simple visual inspection of the exhaust pipe is sufficient to qualitatively determine if urea crystals are present on the surface of the exhaust pipe. However, no methods are currently capable of real-time quantitative measurement of the amount (i.e., thickness) of urea crystals on the surface of the exhaust pipe. Embodiments of the present disclosure are directed to such methods, and systems for performing the methods.